1. Field of the Invention
This invention relates to an element which is used to identify materials, such as, thermoplastics and sort these thermoplastics on a moving conveyor belt. Such structures of this type, generally, employ a diamond knife (or point) for use as an internal reflection element to obtain the mid-infrared spectrum of the material sliced (or punctured) by the knife (or point).
2. Description of the Related Art
It is highly desirable for recycled engineering thermoplastics to be recompounded to produce new formulations which would be almost identical to the original product. In order for this to be successful, the recycle portion must be pure. While contamination with certain polymers may be acceptable, others would produce catastrophic results. Therefore, the basic problem is how to assure purity of a recycle polymer stream.
All recycle proposals involve the grinding of individual items to produce a polymer form that can be economically conveyed, transported, mixed with other materials, and extruded. Identification of individual items, before grinding, is the most efficient process to guarantee purity.
Infrared spectroscopy provides as ideal method of material identification, but sample presentation presents a formidable problem. Often times, these items are too thick to be transparent, too irregular to yield good contact for conventional attenuated total reflectance (ATR) techniques, and/or too dull to reflect a measurable amount of energy. In addition, many items are coated with paint or a metallized or textured layer which would interfere with surface or reflection techniques.
Infrared spectroscopy has proven to be very useful for the identification of polymers. The usual application of this technology involves obtaining a transmission spectrum of the polymer in question and then comparing the spectrum to a library of similarly obtained spectra.
Another typical application involves the use of ATR techniques, as shown in FIG. 1. In particular, light is directed into a high refractive index optical element 2 along light path 8. As the light bounces off the faces of the optical element 2, a portion of the radiation extends beyond the surface of element 2 and penetrates thermoplastic samples 4 and 6. Samples 4 and 6, typically, are cut into strips approximately 1/2.times.13/4 inch. Samples 4 and 6 are mounted and tightened against element 2 with, typically, 15 in.-lbs. of pressure. Each time light ray 8 bounces, some of the radiation extends beyond the face of element 2 as shown in areas 10. These areas 10 of the light are called the evanescent waves 10. As wave 10 extends beyond the face of element 2, wave 10 examines material contacting the surface, namely, samples 4 and 6. In this manner, a spectrum is obtained of a thin section of samples 4 and 6 along the front surface of samples 4 and 6. The thickness examined is from 1-15 microns depending on the material and the refractive index of element 2, the angle of incidence, and the refractive index of samples 4 and 6. Typically, all sample spectra are ratioed against a background spectrum obtained with just element 2.
It is also possible to shorten the process time by comparing conventionally obtained interferograms, instead of, spectra. The problem with this technology is material sampling. In order to get a transmission spectrum, the material must be very thin, usually, on the order of several micrometers to tens of micrometers. The usual way to obtain such a sample involves hot pressing a small sample of the polymer. This is a relatively long and labor intensive process that would be almost impossible to automate on a production line or any conveyor-based process.
In some cases, depending on the thickness of the object, it is possible to obtain mid-to-near infrared spectra without sample preparation. This type of technology has proven useful only for sorting plastic bottles and containers. It is important to note that, in this case, the wall thickness of a typical plastic bottle or container is thin enough to obtain spectra with measurable features, in the near infrared and lower mid-infrared regions, without sample preparation.
In the area of engineering thermoplastics, the majority of recycled pieces are too thick to be transparent to near infrared or infrared radiation. Hence, transmission methods cannot be employed. A technique, based on diffuse reflectance, has been shown to be an effective way to obtain the mid-infrared spectrum for these materials. However, significant sample preparation is required. In particular, a small sample of polymer is removed by abrading with a silicon carbide abrasive paper. The sample, still on the paper as small flakes of polymer, is examined in the diffuse reflectance mode to obtain an infrared spectrum. Again, the spectrum is compared to similarly obtained spectra to identify the material. This technique shows promise for recycle applications only where a single sampling would be required to identify a large number of individual parts. An example of such an application would be the dismantling of a number of identical items where the manufacturer is certain all similar parts were manufactured from the same polymer.
Another technique has been devised to obtain spectrum from a flat, moving sample without contacting the surface. In this case, hot air, directed at a point in the sample, provides localized heating. The resulting infrared emission is collected and analyzed to obtain the spectrum. This technique works well with some samples, but would be expected to present insurmountable problems with the assortment of shapes, sizes, and textures anticipated in a recycle stream of engineering thermoplastics. A more advantageous system, then, would be presented if the optical element were able to avoid the problems associated with thick painted, metallized or textured samples while being able to be employed on a conveyor-based system.
It is apparent from the above that there exists a need in the art for an optical element which is capable of being used on a conveyor-based system, and which at least equals the thermoplastic identification characteristics of the known systems, but which at the same time is capable of identifying thermoplastics which are thick painted, metallized or textured without having to prepare the thermoplastic. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.